Changing the Timeline

06/03/04

A microscopic organism found in China pushes the existence of anatomically symmetrical beings back to about 600 million years.

By Usha Sutliff

Animals - in the form of microscopic yet complex organisms - may have existed millions of years earlier than previous estimates, according to a new study published in the June 3 edition of Science Express.

The study, based on fossils discovered in southern China, pushes the existence of bilaterians - animals that possess the same anatomical symmetry that defines most modern animals, including humans - back to about 600 million years ago, said David Bottjer, one of the study’s lead researchers and a professor of earth sciences at the University of Southern California.

The exact timing of the emergence of bilaterian animals - which have defined front, rear, left and right body surfaces - onto the world stage has long been a topic of debate in the scientific community.

Some trace their beginnings to the “Cambrian Explosion,” a period 20-30 million years long that began roughly 543 million years ago when many of the major groups of animals first appear in the fossil record.

But many paleontologists and molecular biologists have speculated that bilaterian animals are older than that - a theory that may hold true in light of the most recent findings.

“We found some fossils that were a good deal older than what people had found before,” said Bottjer, who teaches in the USC College of Letters, Arts and Sciences. “It doesn’t mean that this is the original [bilaterian], but it is likely from the early beginnings of bilaterian evolution.”

The animal in question is called Vernanimalcula guizhouena, Latin for “small spring animal” - a nod by the discovery team to the “spring” following the so-called “Snowball Earth” time period that ended roughly 600 million years ago when it’s theorized that most of the planet was entombed in ice.

Vernanimalcula guizhouena - which was about the size of four human hairs laid side by side - is thought to have survived that period of extreme cold, Bottjer said.

“It was a little button-shaped organism that probably scooted along the sea floor,” he said. “It had a little mouth, sort of like a vacuum cleaner. It was tiny, but microbes are even smaller so it probably sucked them up so it could eat them.”

Aside from a mouth, Vernanimalcula guizhouena had an anus and paired external pits that the researchers theorized it used to sense environmental conditions, such as light.

Bottjer, along with collaborators from China and Caltech, found the fossils in a landscape of rolling hills, rice farms and phosphate quarries. The team carted a dump truck full of rock from a quarry to nearby Yunnan, where they used rock saws with diamonds embedded in the cutting edge to produce translucent rock slices that were fixed to glass slides and analyzed under a microscope.

From about 10,000 of these translucent slices, they found 10 specimens of the small but surprisingly complex animal.

“What it says is that early in evolution bilaterians may have been very small, but eventually they became large. Humans, which evolved much later, are good examples of large bilaterians,” Bottjer said.

But, he added, the how, why and when have yet to be discovered.

“There’s still more to be found. We expect to find even older stuff,” Bottjer said. “The story is still incomplete.”